The overall goal of this project is to achieve a better understanding of the processes whereby intentions are translated into actions in normally aging adults: so-called 'executive control' processes. Previous behavioral research has suggested significant and progressive reductions in executive abilities in aging adults. The specific aim of this project is to study the executive processes involved in switching from one task to another using high-density electrophysiological recordings (event-related potentials (ERPs)) as our dependent measures. This allows us to directly assess the underlying brain dynamics across the network of areas involved in control processes. Our preliminary data suggest that the deficits in task-switching seen with aging may not in fact be due to prefrontal cortical dysfunction, a commonly held hypothesis, but rather, may result from deficient activation of parietal cortical processes. This has led us to hypothesize that a core deficit in aging may in fact be an inability to sustain attention- a process associated with parietal activations. The experimental paradigm we will use is as follows, and is a modification of a well-established task-switching paradigm, allowing for direct comparisons to the wealth of previous psychophysical data. On every trial, a stimulus comprised of a letter and a number will be presented. For three successive trials, subjects will judge whether the letter is a vowel or a consonant; for the next three trials, they will judge whether the number is even or odd; this sequence will then be repeated. Thus, subjects will be required to switch between a letter-categorization task and a number-categorization task. Their ERPs, as well as their behavioral performance (reaction times [RTs] and error rates), will be recorded. This will allow us to compare subjects' ERPs and behavioral performance on three types of trial: 1) the first trial after subjects have switched from one task to the other ('post-switch' trials); 2) the second trial after a task-switch ('nested' trials); 3) the trial immediately preceding a switch of task ('pre-switch' trials). Thus, the executive processes required to switch will be active before and/or during the 'post-switch' trial. The ERPs associated with these processes will therefore be found a) in the late stages of the 'pre-switch' trials, and/or b) in the very early stages of the 'post-switch' trials. The 'nested' trials provide a baseline against which to compare both the 'pre-switch' and the 'post-switch' trials. The results of these comparisons will shed light on the neural mechanisms underlying the executive processes involved in task switching in both young and normally aging adults. High-density ERP recordings and topographic mapping will permit us to assess the relative contributions of frontal, parietal and earlier sensory areas to these processes.